This invention provides means for measuring on a real time basis a refractive index by dipping directly a probe into a liquid whose refractive index is to be measured. The expected field of application of the present invention primarily includes the adjustment of a refractive index regulating solution and in the field using such a solution.
A refractometer using a refraction method is known as one of the conventional liquid refractometers. This refractometer measures the ratio of an angle of incidence to an angle of refraction at a boundary where the angle of refraction varies. A refractometer using a reflection method is also known. It measures a critical angle at which total reflection occurs at the boundary where the refractive index varies. This is a so-called "Abbe's refractometer". A refractometer using another reflection method determines the refractive index from the coefficient of reflection at the boundary where the refractive index varies.
However, the heretofore known methods described above provide measuring accuracy of refractive index which is as low as from 10.sup.-2 to 10.sup.-3, and hence cannot be used in the field requiring precise control of refractive index such as in the production of an optical communication fiber.
As one of the methods of precisely measuring the refractive index, a method using a so-called "interference method" is known. However, this method cannot measure the object to be measured on a real time basis. In order to control the production process of the optical fiber, the refractive index must be measured instantaneously with a high level of accuracy. Nonetheless, the conventional methods using the interference method essentially require sampling, and none of them can measure the refractive index in situ.